Force transmission device with flat support

ABSTRACT

A pneumatic brake booster has a thrust rod (13) guided by a sleeve (16) supported by a hydraulic piston (14) of a master cylinder (2). The thrust rod has a thrust washer (15) fixedly attached, which bears against a flat bottom of a cup (12) housing a reaction disc (11). The thrust washer is able to slide relative to the flat bottom.

The present invention relates to a force transmission device for anassembly consisting of a pneumatic brake-booster and of a mastercylinder, comprising:

a control rod capable, starting from a resting position, of undergoing amovement in an axial direction under the effect of an input force, thisrod being terminated by a plunger;

a pneumatic piston movable in this axial direction, from a restingposition, under the effect of a pressure differential controlled by theaxial movement of the control rod, this pneumatic piston having an axialannular surface which surrounds the plunger;

a reaction disk housed in a cup with flat bottom and having a free faceonto which the axial annular surface of said pneumatic piston and theplunger are capable of applying output forces;

a substantially axial thrust rod a first end of which receives theseoutput forces and another end of which retransmits them to a hydraulicpiston of the master cylinder; and

retaining means for ensuring that the thrust rod maintains asubstantially axial direction.

Such a device is for example described in U.S. Pat. No. 4,984,506 andwell known in the prior art.

In a general way, brake-boosters and the master cylinders which areassociated therewith are mass-produced and are for this reason subjectto very delicate compromises as far as the cost and the precision ofproduction are concerned.

This leads in particular to major difficulties in obtaining a perfectalignment of the thrust rod with the control rod, without whichdeformations under stress, followed by irreversible damage to thebooster may appear.

The object of the present invention is to provide simple and economicalmeans for avoiding the detrimental consequences of such a lack ofperfect alignment.

To this end, the device of the invention is essentially characterized inthat the thrust rod is securely attached, at its first end, to a flatthrust washer coming to bear, with the possibility of a relativesliding, against the flat bottom of said cup, and in that said retainingmeans are borne by the master cylinder.

In the case where the hydraulic piston of the master cylinder has anaxial cylindrical bore into which the second end of the thrust rodenters, said retaining means preferably comprise a restriction of theinternal diameter of this bore, adapted to limit the possibility of thethrust rod deviating from said axial direction.

The invention will now be described by way of example with reference tothe accompanying single FIGURE which is a view in partial cross sectionof a device according to the invention.

Insofar as the invention relates only to an improvement made topneumatic brake-booster systems, and insofar as the general constitutionand operation of these systems are well known to the person skilled inthe art, these systems will be rapidly recalled here only in order toprovide a total understanding of the improvement which the inventionrepresents.

In a simplified manner, a system of this type comprises a booster 1 anda master cylinder 2.

The booster per se comprises a rigid casing 3 separated into twochambers 3a and 3b, in leaktight manner, by a membrane 4 securelyattached to a pneumatic piston 5 movable inside the casing.

The front chamber 3a, the front face of which is closed in leaktightmanner by the master cylinder 2, is permanently connected to a partialvacuum source (not shown) through a union 6.

The pressure in the back chamber 3b is controlled by a valve 7,controlled by a control rod 8, which is connected to the brake pedal.

When the control rod 8 is in resting position, in this case pulledtoward the right, the valve 7 establishes a communication between thetwo chambers 3a and 3b of the booster.

The back chamber 3b then being subject to the same partial vacuum as thefront chamber 3a, the piston 5 is pushed back toward the right, inresting position, by a spring 9.

The actuation of the control rod 8 toward the left has the effect, in afirst stage, of moving the valve 7 so that it isolates one of thechambers 3a and 3b from the other and then, in a second stage, of movingthis valve so that it opens the back chamber 3b to atmospheric pressure.

The pressure differential between the two chambers then sensed by themembrane 4 exerts on the piston 5 a thrust which tends to move it towardthe left compressing the spring 9.

The braking force which the control rod 8 exerts, through theintermediary of a plunger 10, with the master cylinder as itsdestination, and the brake boost force resulting from the thrust of thepiston 5, are applied onto the upstream (right) face 11a of a reactiondisk 11 housed in a cup 12 adapted to retransmit these forces to athrust rod 13, which has the function of actuating a hydraulic piston 14of the master cylinder.

The invention relates essentially to the force transmission chainconstituted by, together, the control rod 8, the plunger 10, the piston5, the reaction disk 11, the cup 12, the thrust rod 13 and the hydraulicpiston 14.

As shown in the FIGURE, the piston 5 bears against the reaction disk 11through an axial annular surface 5a which surrounds the plunger 10.

The cup 12 is of essentially cylindrical shape and has a flat bottom12a, perpendicular to the longitudinal axis A8 of the control rod, andagainst the entire internal surface of which bears the reaction disk 11.

According to the invention, the thrust rod 13 is, at the first of itsends 13a, securely attached to a flat thrust washer 15 perpendicular tothe longitudinal axis A13 of the thrust rod 13 and coming to bear, withthe possibility of a relative sliding, against the flat bottom 12a ofthe cup 12, and retaining means are provided on the master cylinder 2 soas to ensure that the longitudinal axis A13 of this thrust rod 13remains substantially parallel to the longitudinal axis A8 of thecontrol rod 8.

As the hydraulic piston 14 of the master cylinder generally comprises anaxial cylindrical bore 14a into which the second end 13b of the thrustrod 13 enters, the retaining means may take the form of a sleeve 16inserted into the bore 14a so as to restrict the internal diameterthereof to a value hardly greater than that of the external diameter ofthe thrust rod 13, so as to limit the possibility of the latter adoptinga position in which the directions of the axes A8 and A13 are verydifferent.

By virtue of this disposition, the forces applied by the control rod 8and the pneumatic piston 5 toward the hydraulic piston 14 tend toorientate the axes A8 and A13 parallel to one another, even when thelatter have a slight offset which prevents them from being strictlymerged.

We claim:
 1. A force transmission device of an assembly consisting of apneumatic brake-booster and a master cylinder, comprising:a control rodcapable, starting from a resting position, of undergoing a movement inan axial direction under the effect of an input force, the rod beingterminated by a plunger probe; a pneumatic piston movable in the axialdirection, from a resting position, under the effect of a pressuredifferential controlled by the axial movement of the control rod, thepneumatic piston having an axial annular surface which surrounds theplunger probe; a reaction disk housed in a cup having a flat bottom andhaving a free face onto which the axial annular surface of saidpneumatic piston and the control rod are capable of applying outputforces; a substantially axial thrust rod a first end of which receivesthe output forces and another end of which retransmits the output forcesto a hydraulic piston of the master cylinder; and retaining means forensuring that the thrust rod maintains a substantially axial direction,characterized in that the thrust rod is attached securely, at the firstend, to a flat thrust washer coming to bear, with the possibility of arelative sliding, against the flat bottom of said cup, and in that saidretaining means is borne by the master cylinder.
 2. The device accordingto claim 1, characterized in that the hydraulic piston of the mastercylinder has an axial cylindrical bore into which the second end of thethrust rod enters, said retaining means comprises a restriction of aninternal diameter of the bore, the restriction adapted to limit thepossibility of the thrust rod deviating from said axial direction.